You hit your enemy just right to send them sprawling as if standing on ice.

You may initiate this maneuver whenever an enemy leaves one of your threatened squares in a direction other than directly toward or away from you, before you make an attack of opportunity against them. Make an immediate trip attack against that creature without provoking an attack of opportunity or giving them the chance to trip you if you fail. You may use your weapon's reach instead of your own for this attack. You may also make your normal attack of opportunity afterwards.